Under analysis is a plurality of Light Emitting Diodes (LEDs) as shown in FIG. 1. With reference to FIG. 1, the voltage source may first undergo a voltage waveform transformation (e.g., AC to DC) before driving multiple series blocks of LEDs, where the M series blocks are connected together in parallel. Each of the M series blocks may have circuitry to control current. For conceptual convenience only a single series block of N LEDs is referred to throughout.
Diodes are well known in electrical engineering to be “current-driven” devices. In practice “current-driven” means that a large change in device current is produced by a small change in applied voltage. An ideal diode described in basic literature is the quintessential current-driven device since, in this ideal, an infinitesimal voltage change above a threshold produces infinite current. Real diodes have complicated internal behavior and, of course, finite current. Variations of internal behavior are taken advantage of to produce diodes of different types, such as Zener diodes. LEDs are diodes that produce light but otherwise are built to have common current versus voltage characteristics.
With their characteristic of rapid current increase as voltage rises above a threshold, LEDs are considered to require external current limiting to prevent damage or failure. It has recently been shown, for example in U.S. Pat. No. 6,461,019 “Preferred Embodiment to LED Light String” which is hereby incorporated by reference, that by matching waveform-dependent LED voltages to the stable source voltage, LED current can remain operationally stable without current limiting circuitry. While this “direct drive” approach is simple, inexpensive and electrically efficient, with no added power loss, there are also several drawbacks to this “direct drive” method.
One drawback to “direct drive” of LEDs is that the method of voltage matching requires a stable source voltage—this can often be met with AC line voltage (e.g., 120 VAC) but it may difficult to meet with batteries having large voltage droop. A second drawback to “direct drive” is inherent sensitivity to voltage variations. These voltage variations are of two primary types: source and device. An example of source voltage variation is difference in line voltage between, say, a place in California versus a place in Pennsylvania, where the latter often is several volts closer to the nominal 120 VAC value. Device voltage variations, found in any ensemble of LEDs, are differences in the current versus voltage characteristics from LED to LED caused by manufacturing. An example of device variation is shown in FIG. 2 where a 10% change in device voltage (for 20 mA nominal current) causes about 70% change in device current (for 2.0 VDC nominal voltage). Another drawback to direct drive of LEDs lies in design restriction, where the number N of LEDs used in the series block is strictly determined by voltage matching, with little variation about this number N.
Consider a series block of N LEDs directly driven with AC power, without current control or any voltage transformation in FIG. 1. The voltage and current of this unregulated AC circuit is shown for one period in FIG. 3. As the voltage increases above the diode threshold Vth to its peak value, Vpk, and then falls back down again, the diode current rises sharply in a nonlinear fashion, in accordance to its current versus voltage response, to a peak value, Ipk, and then the LED current falls back down again to zero current in a symmetric fashion. The average LED current, Iavg is obtained by integrating the area under this current spike over the period. With a typical value of average current at 20 mA, the peak current value is around 120 mA and the current pulse has an effective duty factor of around ⅙. This combination of peak current and duty factor for unregulated AC drive is stressful to common LEDs, lowering their reliability and longevity.
The stress due to an AC waveform can be reduced by transforming the voltage. A simple and inexpensive example is bridge rectification, resulting in the voltage and current shown in FIG. 4. Compared to the original AC waveform, peak current is halved to around 60 mA while effective duty factor is doubled to around ⅓; this lower peak value is particularly less stressful to common LEDs. With further waveform smoothing, such as by adding a capacitor across the rectifier output to create rippled DC, peak LED current is closer to average LED current and the devices are stressed even less. Nonetheless, without any LED current control the circuit remains sensitive to voltage variations discussed earlier.
LED current control is often implemented using passive impedance elements, most commonly as resistors. For AC or pulsed drive sometimes reactive components such as capacitors or inductors are considered advantageous for various reasons. This type of LED current control has been the subject of a variety of patents, notably in the early work of Okuno (“Light-emitting diode display, U.S. Pat. No. 4,298,869, Nov. 3, 1981). The primary advantage offered by passive impedance elements is simplicity and cost. However, passive impedance elements are fundamentally soft limiters rather than hard limiters. A soft limiter exhibits current dependence on voltage whereas a hard limiter keeps current constant over a range of voltage.
The need therefore exists for a system for current regulation of a light emitting diodes.